A fuel cell is a mechanism that electrically extracts energy generated when hydrogen is allowed to react with oxygen to produce water, and is seen as clean energy because of its high energy efficiency and the fact that it discharges only water.
An electrode used in a polymer electrolyte fuel cell has a structure including a polymer electrolyte membrane, and on both surfaces of the polymer electrolyte membrane, a catalyst layer formed on a surface of the polymer electrolyte membrane and a gas diffusion layer formed outside the catalyst layer. An electrode substrate is distributed as an individual member that forms the gas diffusion layer in the electrode. As the performance required of the electrode substrate, for example, there are gas diffusibility, electrical conductivity to collect electricity generated in the catalyst layer, and water drainability to efficiently remove moisture generated on the surface of the catalyst layer. To obtain such an electrode substrate, generally, a conductive porous substrate having both gas diffusion ability and electrical conductivity is used.
Specific examples of the conductive porous substrate include carbon felt, carbon paper, and carbon cloth made of carbon fibers. Among them, carbon paper is most preferable from the viewpoint of mechanical strength and the like.
Since the fuel cell is a system that electrically extracts energy generated when hydrogen is allowed to react with oxygen to produce water, under an increased electrical load, that is, under a large current taken to the outside of the cell, a large amount of water (water vapor) is produced. The water vapor condenses into water droplets at low temperature and blocks the pores of the gas diffusion layer, thereby reducing the amount of gas (oxygen or hydrogen) supplied to the catalyst layer. If all the pores are finally blocked, power generation may stop. This phenomenon is called flooding.
To prevent the flooding from occurring as much as possible, conversely, to increase the current value at which the flooding occurs as much as possible, the gas diffusion layer is required to have water drainability. As a means of improving water drainability, an electrode substrate increased in water repellency, which is obtained by subjecting a conductive porous substrate to a water repellent treatment, is usually used.
There has also been proposed a technique to further improve water drainability in which the amount of the water repellent material in the electrode substrate continuously decreases from the catalyst layer side toward the other side (Japanese Patent No. 5079195 and Japanese Patent Laid-open Publication No. 2014-63730).
JP '195 proposes a method of applying a water repellent liquid to a conductive porous substrate while heating the conductive porous substrate to suppress penetration of the water repellent liquid into the conductive porous substrate. If the water repellent liquid is applied while the conductive porous substrate is heated, however, the amount of the water repellent material on the catalyst layer side of the conductive porous substrate becomes insufficient, and water condenses at a site with poor water repellency to inhibit discharge of water out of the system.
JP '730 proposes a method of applying a coating solution made from a carbon powder and a water repellent material to one surface of a conductive porous substrate to make the water repellent material in the coating solution penetrate into the conductive porous substrate. JP '730 proposes that the conductive porous substrate not be subjected to any treatment and the conductive porous substrate is impregnated with a surfactant solution as a pretreatment so that the coating solution is more likely to penetrate into the conductive porous substrate. However, not only the water repellent material, but also the carbon powder in the coating solution penetrates into the conductive porous substrate so that the pores are blocked, gas diffusibility decreases, and power generation performance is deteriorated. In addition, especially when a surfactant is used to perform the pretreatment has a problem that the water repellent material and the carbon powder reach the side of the conductive porous substrate opposite to the side to which the coating solution is applied, and the discharge of water to the outside of the system is inhibited.